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Encyclopedia > IBM 305
IBM 305 at U. S. Army Red River Arsenal
IBM 305 at U. S. Army Red River Arsenal

The IBM RAMAC 305 was the first commercial computer that used magnetic disk storage. IBM introduced it on September 4, 1956. RAMAC stood for "Random Access Method of Accounting and Control." The 305 was one of the last vacuum tube computers that IBM built. The IBM 350 disk system stored 5 million 7-bit characters. It had fifty 24-inch diameter disks. Two independent access arms moved up and down to select a disk and in and out to select a recording track, all under servo control. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for $3,200 per month. More than 1000 systems were built. Production ended in 1961 and the 305 was withdrawn in 1969. Image File history File links BRL61-IBM_305_RAMAC.jpeg IBM 305 RAMAC, Photo by U. S. Army Red River Arsenal File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links BRL61-IBM_305_RAMAC.jpeg IBM 305 RAMAC, Photo by U. S. Army Red River Arsenal File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Disk storage is a group of data storage mechanisms for computers; data are transferred to planar surfaces or disks for temporary or permanent storage. ... International Business Machines Corporation (IBM, or colloquially, Big Blue) NYSE: IBM (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, NY, USA. The company manufactures and sells computer hardware, software, infrastructure services and consulting services. ... September 4 is the 247th day of the year (248th in leap years). ... 1956 (MCMLVI) was a leap year starting on Sunday of the Gregorian calendar. ... RAMAC is an IBM trademark for mass storage products. ... In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ... Magnetic disk storage was a critical component of the computer revolution. ... The term servo can refer to: Servomechanism - usually just shortened to servo, is a device used to effect mechanical motion for a specified distance. ... // Events and trends This map shows two essential global spheres during the Cold War in 1959. ...

Programming the 305 involved not only writing instructions to be stored on the drum memory, but also almost every unit in the system (including the computer itself) was programmed by inserting wire jumpers into a plug-board. hi i am cool xbox is all most as cool as me hi again ... Plug-boards, or, more formally, control panels were devices used to program IBM unit record equipment from roughly the 1930s to the 1960s. ...

A basic system was composed of the following units:

305 Processing unit, containing the magnetic process drum, magnetic core register and electronic logical and arithmetic circuits
350 Disk Storage unit
370 Printer
323 Card Punch
380 Console, containing the Card Reader and IBM Electric typewriter model B1
340 Power Supply


The IBM Electric typewriters were a series of electric typewriters that IBM manufactured, starting in the late 1940s. ...

The 305's architecture

It was a character oriented variable "word" length decimal (BCD) computer with a drum memory rotating at 6000 RPM that held 3200 alphameric characters. A core memory buffer of 100 characters was used for temporary storage during data transfers. Binary-coded decimal (BCD) is, after character encodings, the most common way of encoding decimal digits in computing and in electronic systems. ... hi i am cool xbox is all most as cool as me hi again ... rpm or RPM may mean: revolutions per minute RPM Package Manager (originally called Red Hat Package Manager) RPM (movie) RPM (band), a Brazilian rock band RPM (magazine), a former Canadian music industry magazine In firearms, Rounds Per Minute: how many shots an automatic weapon can fire in one minute On... Generally speaking, the term alphanumeric refers to anything that consists of only letters and numbers. ... A 16×16 cm area core memory plane of 128×128 bits, i. ...

The drum memory was organized into 32 tracks of 100 characters each. Each character was 7 bits, composed of two zone bits (X & O), four BCD bits for the value of the digit, and an odd parity bit (R) in the following format:

 X O 1 2 4 8 R 

Instructions could only be stored on 20 tracks of the drum memory and were fixed length (10 characters), in the following format: An instruction set, or instruction set architecture (ISA), describes the aspects of a computer architecture visible to a programmer, including the native datatypes, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O (if any). ...

T1 A1 B1 T2 A2 B2 M N P Q
Field positions Function
T1 A1 B1 Source operand address – Track, low order AB Character
T2 A2 B2 Destination operand address – Track, low order AB Character
M N Length of operands (each operand must be entirely on its specified track)
P Program exit code; used to select test conditions, perform jumps, and initiate input/output. The 305's plug-board programming determines the action(s) performed.
Q Control code; modifies the operation (similar to an op code), the default operation being a copy from source to destination. Other operations were: "1" Compare, "2" Field compare, "3" Compare & Field compare, "5" Accumulator reset, "6" Blank transfer test, "7" Compress & Expand, "8" Expand, "9" Compress

A computer can perform operations, each of which is assigned a numeric code called an opcode. ...

Fixed-point data "words" could be any size from one decimal digit up to 100 decimal digits, with the X bit of the least significant digit storing the sign (signed magnitude). It has been suggested that Discrete number be merged into this article or section. ... Comparison of Integer Representations in 4-bit In mathematics, negative numbers in any base are represented in the usual way, by prefixing them with a − sign. ...

Data records could be any size from one character up to 100 characters.

Track Specifier Source Function Destination Function
W X Y Z General Storage
0 1 2 3 4 5 6 7 8 9
& A B C D E F G H I
Instruction Storage, General Storage
L Read accumulator Add to accumulator
M Read & clear accumulator Subtract from accumulator
V Multiplicand (1 to 9 characters) or Divisor (1 to 9 characters)
N N/A Multiply (1 to 11 characters)
Stores 2 to 20 character product
in accumulators 0 & 1
P N/A Divide (option)
K 380 Punch Card Input N/A
S T N/A 323 Punch Card Output,
370 Printer Output,
407 Printer Output
Q 380 Inquiry Input/Output
J N/A 350 File Address
R 350 File Data Input/Output
- Core buffer Character selector
$ 382 Paper Tape Input/Output (option)

L and M select the same track, containing ten 10-character "Accumulators". As a destination L specifies addition, M specifies subtraction. (Numbers in these accumulators were stored in ten's complement form, with the X bit of the most significant digit storing the sign. The sign of each accumulator was also held in a relay. However the 305 automatically converted between it's standard signed magnitude format and this format without the need for special programming.) In a CPU, an accumulator is a register in which intermediate results are stored. ... In mathematics and computing, the method of complements is a technique used to subtract one number from another using only addition of positive numbers. ... Relay is also the name of a series of medium-altitude satellites; the first of which was launched in 1962. ...

J, R, and - do not select tracks on the drum, they specify other sources and destinations.


The 305's instruction set does not include any jumps, instead these are programmed on the plug-board:

  • Unconditional jump – the program exit code (P field) specifies a Program exit hub on the plug-board, which is wired by a jumper to a Program entry hub specifying the instruction to jump to.
  • Conditional jump – the program exit code (P field) specifies a Program exit hub on the plug-board, which is wired by a jumper to the appropriate Condition selector common hub to be tested, the two Condition selector output hubs are wired by jumpers to Program entry hubs specifying the instructions to jump to or the Program advance hub to continue in sequence.
  • Multi-way jump – the destination track (T2 field) is set to "-" and the appropriate Character selector hubs on the plug-board are wired by jumpers to Program entry hubs specifying the instructions to jump to or the Program advance hub to continue in sequence.


All timing signals for the 305 were derived from a factory recorded Clock Track on the drum. The clock track contained 816 pulses 12μS apart with a 208μS gap for sync.

Reading or writing a character took 96μS.

The 305's typical instruction took three revolutions of the drum (30mS): one (I phase) to fetch the instruction, one (R phase) to read the source operand and copy it to the core buffer, and one (W phase) to write the destination operand from the core buffer. If the P field (Program exit code) was not blank, then two (D phase and P phase) additional revolutions of the drum (20mS) were added to the execution time to allow relays to be picked. The Improved Processing Speed option could be installed that allowed the three instruction phases (IRW) to immediately follow each other instead of waiting for the next revolution to start; with this option and well optimized code and operand placement a typical instruction could execute in as little as one revolution of the drum (10ms).

Certain instructions though took far longer than the typical 30mS to 50mS. For example, multiply took six to nineteen revolutions of the drum (60mS to 190mS) and divide (an option) took ten to thirty seven revolutions of the drum (100mS to 370mS). Input/Output instructions could interlock the processor for as many revolutions of the drum as needed by the hardware.

External links

  Results from FactBites:
IBM 305 RAMAC- The First Computer with a Hard Disk Drive in 1956 (230 words)
IBM introduced the 305 RAMAC computer on September 13th, 1956, which was the first computer to include a disk drive named the IBM 350 Disk File.
IBM leased the 350 Disk File for a $35,000 annual fee.
At that time the Instant Replay Deck was introduced, which permitted the storage and playback of brief segments of analog video.
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